On-line normalization process of seamless steel pipe
On-line normalization process of seamless steel pipe
Online normalization is a heat treatment process developed in recent years, that is, deformation normalization. It is the steel tube after rolling of the continuous rolling tube, cooled to below the recrystallization temperature on the cooling bed, and then enters the reheating furnace. After the steel tube is heated to the temperature above Ac3 or Acm in the furnace, it is kept for a period of time to make the steel metallographic The structure is transformed into austenite, and then out of the furnace. After being reduced and rolled by a sizing machine or a tension reducer, the austenite structure is transformed into pearlite, which is a modern control. Rolling scheme.
Performance requirements for online normalization:
While meeting the strength requirements, it achieves an excellent match of toughness and strength, has good weldability, and excellent low temperature toughness.
Purpose of online normalization:
(1) The structure of the steel becomes uniform and the grain is refined;
(2) Improve the mechanical properties of some steel grades;
(3) Improve the metallographic structure and properties of low carbon steel and low alloy steel, and create conditions for the diffusion of alloying elements.
The structure of ferrite plus pearlite appears under a metallographic microscope. If it is a microalloyed steel, the precipitated phase and second phase particles can be observed under a transmission electron microscope.
On-line normalization is to insert a normalizing process in the middle of the rolling process, which uses part of the rolling waste heat, which can shorten the normalizing heating time. Microalloying online normalizing steel replaces offline normalizing or modulating steel, which reduces the costs related to heat treatment, finishing, energy, decarburization, and iron loss (oxidation). In short, online normalization not only simplifies the process, but also saves energy.
Compared with quenched and tempered steel, on-line normalized steel has better cutting performance, because the ferrite + pearlite structure is better than the tempered sorbite structure.
Process description for online normalization:
After the rolled steel is cooled to below Ar1, all austenite is transformed into ferrite + pearlite structure, that is, after reheating and all transformation into austenite, air cooling obtains finer ferrite + pearlite organization.
Determine the cooling time before entering the reheating furnace according to the size of the workpiece, ensure that it is cooled below the Ar1 temperature, and determine the time the workpiece is in the reheating furnace to ensure that the alloy is dissolved and the structure is all transformed into austenite and homogenized. In addition, Set the reheat furnace temperature.
In order to dissolve all the alloy elements into austenite and homogenize the austenite, the workpiece in the furnace should be kept within the specified heating temperature range for an appropriate time. The length of time depends on the effective thickness, steel type, and equipment of the workpiece. Furnace method, furnace loading volume, furnace loading temperature, furnace performance and sealing degree are related to factors.
Different cooling methods mean that the workpiece cooling rate is different, such as stack cooling, side-by-side cooling, workpiece space cooling, and single cooling. The cooling rate changes from small to large. Generally speaking, the grain size decreases, the percentage of pearlite increases, and the pearlescent The distance between the body pieces is reduced, which improves the strength and impact toughness.
The holding time of the workpiece in the reheating furnace is an indispensable control parameter. Insufficient heat preservation time, insufficient strength, excessive heat preservation time, wastes energy, reduces production efficiency, and may cause decarburization on the surface of the workpiece.
Before the on-line normalized steel enters the reheating furnace, it has undergone a staged rolling process. Each part of the workpiece must complete the transformation of austenite to ferrite + pearlite, so the part where the workpiece is cooled relatively slowly It must also be cooled below Ar1.
The temperature control of the reheating furnace is very important to the performance of steel. The reheating furnace raises the temperature of the workpiece, not only to ensure the necessary structural transformation and diffusion, but also to make all the necessary alloying elements melt into the austenite. The temperature is low, the strength is insufficient, the temperature is high, the toughness decreases, and the temperature is too high, the strength will also decrease.
Microalloying elements are added to steel for two purposes, namely grain refinement and precipitation strengthening. Both effects are caused by the precipitation of microalloy carbides, nitrides or carbonitrides.
Reasons for bending in online normalization:
When the cooling bed is bent (before the reheating furnace), the steel pipe itself has inconsistent structural transformations, which is the cause of the bending. Some steel grades not only get ferrite + pearlite, but also produce a part of bainite structure at a certain cooling rate. The body structure stress is relatively large, the steel pipe is locally cooled first, and the part where bainite is generated first may be bent.
Bending occurs after entering the reheating furnace. This is because the temperature of the steel pipe inlet in the reheating furnace is difficult to maintain the same temperature with other parts. When the temperature of the pipe entering the furnace is too low, the higher furnace temperature in the furnace will increase the thermal stress of the pipe sharply. When the furnace condition is not ideal, the pipe Due to the inconsistency of the temperature in the furnace, the thermal stress of different parts will be quite different. This causes the tube to bend.
